Method of heat treating and forming an aluminum sheet



United States Patent 3,379,580 METHGD OF HEAT TREATING AND FORMING AN ALUMINUM SHEET Paul P. Zeigler, Spokane, Wash, assignor to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware No Drawing. Filed Sept. 20, 1965, Ser. No. 488,715

4 Claims. (Cl. 14811.5)

ABSTRACT OF THE DISCLOSURE An alloy specially adapted to color anodizing by close control of composition and method of preparation. The alloy contains from 0.090.l5% silicon, from OAS-0.65% iron, from 0.03-0.07% copper, from 0.70.9% magnesium, from 0.050.1% chromium and a maximum of 0.04% manganese and 0.25% zinc.

The alloy is prepared by solution heating between 1100ll50 R, cooling to below 950 F. at a maximum rate of 30 F. per hour, cooling to below 250 F. at any rate, and reheated to a temperature below 950 F. for rolling.

This invention relates to an aluminum alloy and a method for treating it which particularly adapts the alloy to accept an inherently colored anodic oxide coating.

The process for producing inherently colored anodic oxide coatings on aluminum and aluminum alloys is now familiar. The coatings are produced by immersing the aluminum article to be anodized as the anode in an electric circuit which includes the anode, an inert cathode, and an electrolyte containing sulfuric acid and any of certain substituted aromatic sulfonic acids. The aromatic sulfonic acids having particular utility are sulfosalicylic acid and sulfophthalic acid although others have been found to be operative. Although sulfuric acid is the most useful material to provide sulfate ions, the anodizing process may be carried out with heavy metal sulfates dissolved in the electrolyte instead of or in addition to sulfuric acid, the most common of which is ferric sulfate. The anodic oxide coatings obtained by this process range in color from gold to black and may be made with almost any intermediate shade of bronze or brown. Such color anodizing processes have gained wide acceptance for producing aluminum for architectural and other decorative uses.

The process of producing inherently colored anodized aluminum articles has created problems which were not known to the anodizing art when only colorless coatings were produced. Chief among these problems is reliable reproduction of colors and other appearance factors. For example, an architect may choose colored aluminum for a structure from small samples, after which an anodizer must make thousands of square feet of anodized aluminum in the form of large panels, and these panels must match the sample as well as each other in order to be acceptable for use in the structure. During the course of making a. large number of such articles the electrolyte composition or other processing variables may change, and such changes may affect the appearance of the product. Maintaining processing variables andelectrolyte composition constant, although not a simple matter, is a commercially feasible one from the anodizers point of view and attention to these details can mitigate changes in the appearance of the product.

However, one element that is beyond the anodizers control is the color response an alloy has to a color-anodizing treatment that results from metallurgical considerations. For example, two specimens of a given aluminum alloy may have slightly differently colored anodic oxide coatings produced on them by identical anodizing processes if,

3,379,580 Patented Apr. 23, 1968 for example, one of these alloys has an impurity-level composition different from the other, or even if two alloys with idential composition were subjected to different thermal treatments during the course of their production. For instance, differences in the homogenizing temperature of the rolling ingots, or differences in time held at the homogenizing temperature, or differences in the rate of cooling ingots after homogenizing have been known to produce significant differences in color response when those ingots were finally rolled into sheets and subjected to color anodizing. Moreover, appearance differences arising from slight differences in impur-i-tydevel may be exaggerated by variations in thermal treatment. I

In making sheet material for color anodizing, alloys designated by the Aluminum Association classification system as 5005 have demonstrated desirable properties with respect to their ability to produce colored anodic oxides, and their ability to be formed into smooth sheets with physical properties adequate for architectural uses. The Aluminum Association composition designations for 5005 alloy are:

Others (total) Aluminum Association designations which do not show a range indicate maximum permissible composition. It has been found that composition variations within the above given ranges are not permissible for alloys used in color anodizing in that certain of the components cause large color response changes so that the difference between maximum and minimum compositions create alloys which, when subjected to identical anodizing treatments, will produce colors too different from one another to be acceptable. Further, it has been found that the presence of too much of some of the impurity elements e.g. Mn, Cr, Fe, Si causes the alloy to be extremely sensitive to its thermal history with regard to color response. That is, certain impurity elements will cause the alloy to respond to color anodizing substantially differently if thermally treated in one Way than if thermally treated in a different way.

This invention includes the discovery of composition limits for aluminum alloys used for producing inherently colored anodic oxide coatings, and thermal treatments for those alloys which create a sheet material that is not sensitive either to reasonable variations in composition or in thermal history. Therefore, this invention provides aluminum which is easy to anodize with reproducible and predictable appearance and aluminum which has these anodizing response properties locked in by mill practice so that anodizers, who are usually poorly equipped to carry out metallurgical processes, can simply anodize the material in the as-received thermally-treated condition and rely on the appearance-response that wiil be produced from the anodizing process.

The invention includes constituting an aluminum alloy which consists essentially of aluminum, from about 0.06- 0.15% silicon, from about OAS-0.65% iron, from about 0.03-0.10% copper, up to about 0.07% manganese, from 07-09% magnesium, from about 0.05 to about 0.10% chromium, up to about 0.25% zinc and up to about 0.15% miscellaneous impurities. All compositions stated in the specification and appended claims are on a weight basis. The alloy thus constituted is cast as an ingot and heated to homogenizing temperature, preferably between 1100 and 1150 P. where it is maintained until most of the soluble elements enter solid solution, preferably for a period of at least 7 hours. The objective of this treatment is to produce a metallurgical structure in which the primary alloying ingredients and impurities are uniformly dispersed or dissolved in the aluminum. The alloy is then cooled slowly to a temperature lower than 950 F. the cooling being at an average rate of 30 F. per hour or slower. After the temperature of 950 F. is reached the ingot can be cooled to room temperature at substantially any rate and it may then be scalped or otherwise prepared for rolling. Before rolling into the form of sheet, the ingot is reheated to a temperature below 950 F. preferably from 850-950 F. and maintained at that temperature for about 1 hour. Rolling may then be performed in the usual manner to produce sheet or plate of any desired thickness.

It is presently believed that milkiness or cloudiness in anodic oxide coatings is caused by particles of precipitated alloying elements or combinations of alloying elements. In normal thermal treatments where an alloy is homogenized and removed from an oven to air cool, the cooling rate from homogenizing temperature is so rapid that the alloy quickly reaches a temperature at which diffusion of atoms or molecules within the alloy is very slow. As a result, precipitation is in the form of many small particles, and these many small particles will appear in a later produced anodic oxide coating where they will scatter light to give the effect of a cloudy or milky coating. Among the first particles to precipitate as the temperature of an alloy drops below the homogenizing temperature are the iron-containing particles which are usually combinations of iron with aluminum, manganese, chromium and other ingredients of the alloy.

In the proces of the present invention the time spent between homogenizing temperature where all of the soluble ingredients are either in solid solution or dissolving, and 950 R, where the diffusion rate for iron-bearing particles becomes too slow to permit significant growth of particles of precipitate, is prolonged enough to permit the iron-containing precipitate particles to form as relatively large particles. The same total amount of precipitation occurs whether in small particles or large particles and the volume of a particle is proportional to the third power of its diameter, and it is therefore evident that if particles of precipitate are permitted to grow significantly larger, an alloy will contain many fewer particles. It has been found that there is much less scattering of light when few large particles are in an anodic oxide film than when many small particles are, and as a result an anodic oxide film containing fewer and larger particles will appear clear and rich even though the same amount of precipitate is present within the film.

Illustrations of the practice and benefits of this invention are given in the following examples, which compare the practice of the invention (Example 2) with the former practice (Example 1):

Example 1.Over an extended period, about 500 lots of 5005 alloy were cast and rolled. Metal compositions of these lots were within the following limits, expressed in percent:

Percent Si 0.15-0.30 Fe 0.50-0.70 Cu 0.03-0.13 Mn (max.) 0.10 Mg 0.70-0.90 Cr (max.) 0.10 Zn (max.) 0.25 Others (each, max.) 0.05 Others (total, max.) 0.15

All ingots were homogenized by holding for one to three hour-s at 1000-1025 F. and then cooling in air to room temperature, without special control of the rate of cool- 4 ing, and then scalped. For hot rolling, the scalped ingots were then reheated to 875-950 F. for one hour minimum and rolled. Cold rolling practice and intermediate anneals varied according to the final temper desired and generally followed established plant practice. After etching in 5% by weight aqueous caustic soda solution for 10 minutes at 130 F., sheet samples from each -lot were anodized in an aqueous bath containing -100 g./l. of sulfosalicylic acid and 4.5-5.5 g./l. of sulfuric acid at 77 F. with a beginning anode current density of 24 amp/sq. ft. When the anodizing cell voltage reached 50 volts, anodizing was continued at that voltage until 10.5 ampere-hours per square foot of electricity had passed through the sheet. The anodized sheets were then immersed for 20 minutes at 195-205 F. in an aqueous bath containing 1.0 g./l. of sodium lignosulfonate and 0.5 g./l. of nickel acetate at pH 50:50.2. The color of each anodized sheet was measured according to the CIE system of color measurement. Green reflectance, an indication of the lightness or darkness of the surface, was measured by the percentage of incident green light reflected by the surface. Yellowness factor, as defined by the term Amber Reflectance-Blue Reflectance Green Reflectance X Si 0.06-0.15 Fe 0.47-0.61 Ou 0.05-0.08 Mn 0.01-0.07 Mg 0.75-0.87 Cr 0.05-0.08 Zn (max.) I 0.25 Others (each, max.) 0.05 Others (total, max.) 0.15

All ingots were homogenized by holding for not less than nine hours at 1100-1150 F. and cooled at an average rate of 30 F. per hour, or slower, to 950 F. The ingots were then air-cooled to room temperature and scalped. For hot-rolling the ingots were reheated to 850-950 F., held for at least one hour, and then rolled. Cold rolling and annealing practice were as in Example 1. All sheet samples from this example were then etched, anodized and sealed as in Example 1 and the color factors measured. The green reflectances ranged from 19 to 27 inclusive and the yellowness factors from 22 to 29 inclusive. This represents a substantially smaller color range than in Example 1 and would be the preferred material for architectural use.

In addition to the narrower range of anodized color shown by the material of this invention there is an added benefit of noticeably more glossy pleasant appearing anodic coatings. Moreover, clear anodic coatings produced by sulfuric acid anodizing of the material of this invention also show asimila-r pleasant gloss, in contrast to the matte appearance of similarly anodized material of Example 1. Furthermore, by extended etching of the material of this invention, followed by sulfuric acid anodizing, a desirable white appearance can be imparted to the surface.

The alloy constituted, thermally treated and rolled as set forth above has the following properties. It is a 5005 alloy with desirable properties of strength, fabricability,

good surface characteristics, and good anodizing characteristics. The alloy is not unduly sensitive to variations in thermal history. That is, two ingots having any thermal histories within the ranges characteristic of this invention will have substantially the same response to a color anodizing treatment. The material constituted as above is not sensitive to variations in composition, within the stated limits. That is, two ingots having any composition characteristic of this invention will have substantially the same response to a color anodizing treatment. The compositions and thermal treatments specified above lockin the color anodizing response of the sheet so that anodizers can rely on the color response of the alloy they receive and nee-d only concentrate on the details of the electrolytic anodizing function that they perform to insure reproducible colors in their product.

What is claimed is:

1. The method of preparing aluminum sheet for color anodizing which comprises:

'(A) constituting an alloy consisting essentially of from about 0.06-0.15% silicon, from about OAS-0.65% iron, from about 0.03-0.10% copper, not more than 0.07% manganese, from about 0.7-0.9% magnesium, from about ODS-0.1% chromium, not more than 0.25% zinc, not more than 0.15% other ingredients and the balance aluminum,

(B) heating the alloy thus constituted to homogenizing temperature and maintaining it at said' temperature for a period sufiicient to place most of the soluble ingredients into solid solution,

(C) cooling the alloy from homogenizing temperature to a temperature below about 950 F. at an average rate of less than 30 F. per hour, and

(D) rolling the resultant alloy to form a sheet.

2. The process set forth in claim 1 wherein said homogenizing temperature is from about 1100-1150 F.

3. The method of claim 1 wherein the alloy is cooled from 950 F. to room temperature, scalped, and reheated to a temperature of from about 850950 F. and subsequently rolled to form a sheet.

4. The method of preparing aluminum sheet for color anodizing which comprises:

(A) constituting an alloy consisting essentially of from about 0.06-0.15% silicon, from about 0.45 to about 0.65% iron, from about 0.03-0.10% copper, not more than 0.07% manganese, from about 0.7-0.9% magnesium, from about 0.050.l0'% chromium, not more than 0.25% zinc, not more than 0.15 other ingredients and the balance aluminum,

(B) heating the alloy thus constituted to a temperature of from about 1100-1150 F. and maintaining it at that temperature for a period of at least 9 hours,

(C) cooling the alloy to a temperature of 950 F. at

an average rate of less than 30 F. per hour,

(D) further cooling this alloy to a temperature lower than 220 F.,

(E) reheating the alloy to a temperature of from about 850950 F. and maintaining it at that temperature for at least 1 hour, and

(F) rolling the resultant alloy to form a sheet.

References Cited UNITED STATES PATENTS Re. 26,216 5/1967 English 14831.5 2,814,576 11/1957 Zickendraht et al. 1486.1

OTHER REFERENCES Metals Handbook, 8th edition, p. 942, published by the American Society for Metals, received in Scientific Library in 1961.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

W. W. STALLARD, Assistant Examiner. 

